This invention relates to refrigeration or cooling controls, and in particular to frost sensors in those controls which employ thermistors as frost sensors elements.
Frost build-up on the cooling coils of a refrigerator or heat pump is one of the major causes of loss of efficiency and energy waste in these devices. It has thus become common for these devices to include defrosting means, including controls which energize these defrosting means in regular intervals. If for some reason, however, frost has not built up to the extent requiring defrosting, the defrost means energizes anyway, wasting more energy. A further control is therefore needed to determine whether sufficient frost is present to require a defrost cycle.
The use of a thermistor simply as a temperature sensor is well known. For example, Kusuda, U.S. Pat. No. 3,578,754, issued May 18, 1971, discloses a defrosting controller for electric refrigerators which employs a thermistor as a temperature sensor. Similarly, Sutton U.S. Pat. No. 3,248,892, issued May 3, 1976, employs a thermistor as a frost sensor, in that when the temperature of the thermistor decreases sufficiently, due to its being covered by frost, a defrost element is activated. This design has the disadvantage, however, that the thermistor is continuously energized in order to continuously sense the amount of frost. The thermistor sensor is thus continuously conducting current, resulting in an increase in the temperature of the thermistor and a distortion of its indication of frost. Therefore, a more accurate circuit for sensing frost is needed.